Abstract: By using the ratio of the G component to the R component G/R, or the ratio of the B component to the R component B/R, from a television monitor image, and referring to a conversion table for converting to distribution temperature, a surface temperature of a flame is found for each pixel. An area in which luminance signals included in an image of a surveillance area exceed a prescribed level is sampled as a flame outline, and at least the distribution temperature of a sampled flame region is detected, and distance to a radiant energy source is measured. The amount of radiant energy from the radiant energy source is estimated by computation, based on the sampled flame area and its distribution temperature and the distance to the source of radiant energy.